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Frank - The first quote is speaking of flow from the cylinder back into the head's flow tract. The "shrouding" it reads of occurs in the combustion chamber design, not the area behind the valve.

The second quote actually says exactly what I said. Straighter is better.

Shawn D - The "Bag of snakes" you refer to is an equal length header which is designed to improve pulse scavenging. Reading back through my replies to Frank and theunchosen, you can see that this has been addressed - No, a straight exhaust will not be able to scavenge flow in the same way that today's exhaust systems do, however, pulse scavenging can still be acheived by tuning the length/volume of the exhaust. (Pressure waves do the same thing on each side of the engine. Timing them creates either positive or negative pressure at the valve face.)

Before saying that a particular design "must be the most efficient", you should look into why it is used. Those headers can be tuned for length and volume to shape the torque curve.

It's a given that "less restrictive bends" will be more beneficial than sharp radii in the exhaust flow - think about that though. The "less restrictive bends" are commonly of less radius, and progressively, the less radius the bends have, the more velocity the flow has. It's a widely known, commonly accepted fact of fluid dynamics.

Common practice is to widen the radius of existing bends, so that the final product still fits within packaging constraints, but has better flow characteristics. A wider flow path creates a straighter available line in the exhaust flow. Straighter is better.

By tuning the cross sectional area of a flow tract, you can increase or decrease velocity, based on the volume of fluid you need to flow. Obviously there is an optimum flow volume per shape/size of exhaust pipe. This is the reason that the best tuned exhausts will still only create gains in certain parts of the RPM range. They can create a specific resonant gain based on volume manipulation, and they can create an optimum shape/size for a given volume of flow, decreasing pressure at the valve face by increasing velocity for that range of flow.

To address engines needing back pressure - Um... prove it. It is commonly accepted that engines of any common type do NOT need back pressure to operate. Pressure at the valve's backside cone only allows for the valve to be partially opened before flow begins exiting the cylinder. Providing an optimum exhaust means utilizing negative pressure at the valve's rear face to increase VE, so that the engine doesn't need to provide any power to expel the exhaust under pressure.

Here's an experiment - apply vacuum to a non-running engine. When the vacuum gets strong enough, the piston will rise. This means that the engine would have had to expel no energy in releasing the exhaust gasses. This would increase volumetric efficiency. Obviously, you're not going to create vacuum at the exhaust without added energy, but the higher the pressure differential across the valve, the faster the exhaust can exit, with less energy needed to do so.